Plastic films are used in areas of magnetic recording, agriculture, packaging, building materials, and the like, in which demand is great, because of advantages in properties, e.g., the strength, the durability, the transparency, the flexibility, and impartation of surface properties. Most of all, biaxially oriented polyester films are used in various areas because of excellent mechanical properties, thermal properties, electrical properties, and chemical resistance. In particular, the usefulness for magnetic recording media is far ahead of other films. However, the dimensional stability and the heat resistance of the polyester film are inadequate depending on uses and, therefore, there is a limitation of application to various industrial material films. For example, in the magnetic recording medium use, reduction in thickness and higher density recording are pursued for miniaturization and long-duration recording, and requirements for higher strength and improvement of form stability and thermal dimensional stability in a use environment of a base film have become even more intensified. In the circuit material use and the like, requirements for films having highly balanced heat resistance, thermal dimensional stability, mechanical strength, chemical resistance, and the like have been intensified in accordance with the needs for miniaturization and more functionality in electrical and electronic areas.
Research was previously conducted on methods in which polyesters were allowed to contain particles and, thereby, fibers and resin moldings were allowed to have high strength. As a result, a method in which fibers have been allowed to contain metal oxide fine particles having particle diameters of 100 nm or less and, thereby, the strength and the dimensional stability of the fibers have been improved (Japanese Unexamined Patent Application Publication No. 1-192820), a method in which a metal complex coordinated with glycols has been added to a polyester-based resin by polymerization, the metal has been reduced to an element, and fine particles of a metal, e.g., palladium, have been finely dispersed into a polymer so as to allow a resin molding to have a high modulus of elasticity (Japanese Unexamined Patent Application Publication No. 10-298409), and the like have been proposed. However, there is no example in which these technologies are applied to films and, in contrast to the present invention, it was not an object to raise the melting point of the film, and to improve the heat resistance and the thermal dimensional stability of the film.
Research was conducted on methods in which biaxially oriented polyester films were allowed to contain particles. Examples of methods include a method in which particles having particle diameters of less than 300 nm of oxides of fifth and sixth period elements in the periodic table of elements (Japanese Unexamined Patent Application Publication No. 3-115437 and the like) are contained in a film. It is a primary object of those proposed methods to form a film surface and improve the scratch resistance, and it is not an object to improve the heat resistance, the thermal dimensional stability, and the mechanical properties of the film.
It would therefore be advantageous to provide a high-quality biaxially oriented thermoplastic resin film having excellent heat resistance, thermal dimensional stability, and mechanical properties. In particular, it would be advantageous to provide a biaxially oriented thermoplastic resin film which significantly improves practical properties which have been taken seriously in various uses. Example of properties to be improved include the running durability in the magnetic recording medium use, the flatness and the warp during processing in the circuit material use, the heat resistance in the capacitor use, shift of printing in the ribbon use, and shift of a circuit in the card use.